This invention relates to an improved process for adding magnesium oxide to polymeric compositions based on vinylidene chloride copolymers.
Copolymers of vinylidene chloride with such copolymerizable monomers as acrylonitrile, vinyl chloride and lower alkyl acrylates have found use in the manufacture of filaments, sheets, tubes, films and extruded and molded shapes. It is known that such polymers are difficult to work with due to their brittleness, relatively poor melt flow and susceptibility to thermal degradation, as evidenced by the development of discoloration and by the generation of gas bubbles during fabrication of said polymers, and particularly during extrusion of film materials from said polymers. It has, heretofore, been common practice to incorporate plasticizers into vinylidene chloride polymer compositions to improve workability, or to reduce brittleness, of the polymer compositions. Thus, citric acid esters (see British Pat. No. 739,411), sebacic acid esters (see U.S. Pat. No. 2,604,458) and/or phthalate esters (see British Pat. No. 811,532) have been utilized in combination with vinylidene chloride polymers to produce film materials. Polymer compositions containing such plasticizers in conjunction with various stabilizer systems, e.g., combinations of an epoxidized soybean oil and an oxide of the metals of Group II of the periodic table, such as magnesium oxide, have also been used as disclosed, e.g., in U.S. Pat. No. 3,261,793 wherein a rubber-like copolymer compatible with the vinylidene chloride polymer was added to impart low temperature flexibility.
It is known that these plasticized vinylidene chloride copolymer compositions are less than wholly satisfactory for a number of reasons, e.g., most of the known plasticizers have poor compatibility with vinylidene chloride copolymer compositions and migrate to the surface of articles, such as films, produced therefrom. Plasticizer migration is especially disadvantageous where the polymeric film material is used in food wrapping applications. Further, the presence of such plasticizers often significantly reduces the tensile strength and tends to increase gas permeability, particularly permeability to water vapor and air, of polymeric films produced from the vinylidene chloride copolymers. As such, the addition of plasticizers to vinylidene chloride copolymer compositions to produce polymeric film material is not without problems. However, when film material is produced from normally crystalline vinylidene chloride copolymer compositions without the use of plasticizers, the resultant film material is frequently characterized by the presence of gas bubbles. Gas bubbles are detrimental where clear, continuous film material is required. It is believed that the gas bubbles result when the vinylidene chloride copolymer degrades via dehydrohalogenation to evolve gaseous hydrogen chloride.
A solution to the problem of gas bubbles has been disclosed in U.S. Pat. No. 3,891,598 wherein substantially plasticizer-free extruded film material comprising a blend of a normally crystalline vinylidene chloride interpolymer with small amounts of magnesium oxide was prepared in a process comprising the steps of admixing magnesium oxide in dry powdered form with dry powdered vinylidene chloride polymer to form an admixture; passing the admixture through a screen to remove any agglomerates having a diameter of greater than about 500 microns from the admixture; and extruding and stretching the admixture into film form. A special technique of adding the magnesium oxide to the vinylidene chloride polymer resin was devised to counteract the tendency of the magnesium oxide particles to cling to metal surfaces and to thereafter form agglomerates which subsequently break off into the extruder melt and result in holes in film formed from the extruded and stretched admixture. The special technique involved conducting the magnesium oxide powder via a flexible tube under the surface of dry vinylidene chloride copolymer particles while such polymeric particles were in motion in a ribbon or cone blender. However, stretched films so produced are often characterized by the presence of small white specks or agglomerates of magnesium oxide and frequently by the presence of small holes. U.S. Pat. No. 3,891,598 also discloses the use of ethylene vinylalkanoate copolymers, such as ethylene vinylacetate, to provide added strength and/or flexibility noting that when adding such materials it is preferable to add the ethylene vinylalkanoate copolymer to a blend of magnesium oxide and vinylidene chloride after the blend has been screened to remove agglomerates.
It is, therefore, the primary object of the present invention to produce vinylidene chloride polymer film materials characterized by reduced permeability to gases such as water vapor and air and which, in addition, can be thermally fabricated, e.g., extruded and stretched into film form, with the substantial absence of discoloration, bubbles, holes and agglomerates of magnesium oxide particles.
Additional related objects will become apparent from the following specification and claims.